An adaptive resource allocation for multiaccess MIMO/OFDM systems was developed
to enhance power efficiency by exploiting multiuser diversity, and channel variations
in the time, frequency, and space domains [1]. However, by increasing the number
of antennas, the scheme has a limitation in the affordable maximum data rate due to constraints
on the maximum modulation order even without transmit power constraint. More
antennas should allow more users to share the same subcarriers such that total affordable
maximum data rates should increase accordingly. The paper presents an enhanced version
of the scheme to remedy this problem. After the subcarrier assignment, the proposed
scheme relaxes the maximum modulation order constraint in the bit loading process and
presents an iterative bit swapping procedure, which are the two points deviated from the
scheme in [1] and solve the related above-mentioned problem. A model of the multi-user
channel on uplink with subcarrier reuse is considered. The proposed scheme aims to
minimize the total user transmit power while satisfying the required data rates, the
maximum transmit power constraint, and the bit error rate of each user. The proposed algorithm
offers better performance than the high complexity greedy multi-user algorithm
as illustrated in the simulation results. 0-35% improvement in terms of percentage in
meeting the required data rates is observed from the simulation results and 0-5 dB
transmit power gain is achieved.

Received November 19, 2008; revised March 26, 2009; accepted June 30, 2009.
Communicated by Chung-Ta King.
* This work was supported by the National Science Council of Taiwan, R.O.C. under contract No. NSC 99-
2221-E-008-039. This paper appeared in part at the IEEE International Workshop on Antenna Technology,
Chiba, Japan, March, 2008.